The technique of casting polymeric articles is old and well established. In general, there are two approaches to casting. One of these is casting either a monomeric or partially polymerized syrup into a mold or shape and conveying this into an oven or autoclave in order to finish polymerizing the article with a temperature treatment. The other general type of casting involves solution casting which is also a long utilized technique for producing plastic film and sheet materials.
The general technique of solution casting involves forming a solution of the film-forming polymer into a suitable solvent, casting the resulting solution on a suitable substrate, evaporating the solvent and winding the resultant film on rolls.
Usually solvent recovery systems are employed in order to recover the solvent and minimize the loss of an expensive process component.
Solution cast opaque films have been conventionally prepared by adding pigments, fillers, flame retardants and solubilizers to a solution of the film-forming material, which pigment acts as an opacifying agent. Without such an agent such film would be colorless or transparent. Opacifying agents often embrittle the film.
Various processes have been described in the art for preparing opaque films which rely for opacity upon the presence of a large number of voids in the film. Such films may be prepared by depositing a film from and emulsion, i.e., either an oil-in-water or a water-in-oil emulsion.
When a water-in-oil emulsion is used--i.e., one in which minute droplets of water are dispersed in a continuous phase of a film forming material--the emulsion is deposited as a coating and the organic solvent which comprises the continuous phase of the emulsion is evaporated therefrom. This causes gelation of the film-forming material and entrapment of the dispersed water droplets. The water is then evaporated leaving microscopic voids throughout the film structure.
Still another technique for obtaining a porous, opaque, nonpigmented film is set forth in U.S. Pat. No. 3,031,328. Basically, this process contemplates preparing a solution of a thermoplastic polymer material in a mixture of a volatile organic solvent and a volatile non-solvent liquid which has an evaporation rate substantially less than that of the solvent. The clear homogeneous solution is then coated on a suitable backing material and dried by evaporation to produce an opaque blushed film which is adapted to be rendered locally transparent by heat or pressure. These films are useful as recording films.
Other techniques for forming opaque, porous, nonpigmented, microporous thermosetting films are set forth in U.S. Pat. No. 3,655,591.
Nevertheless, in spite of the above, the art has never appreciated the unique articles which result when a specific type of polymer is cast in a certain manner to produce an essentially non-porous, non-foam microcellular structure which has unique and unusual properties and is incidentally opaque. The art has concentrated on techniques wherein the opaqueness is the sine qua non of the structure and the other properties are not of significance.